The print engine on printers and electrophotographic copy machines operates by forming a latent image on a photoconductive belt, depositing toner on the photoconductive belt, and then developing and transferring the developed image to an image receptor. There are a number of parameters in the print engine that are critical in providing high quality copies. One of these is the density of the toner that is applied to the photoconductive belt. The density is a function of the voltage that is imparted to the photoconductive belt and the exposure levels of the image. In particular, it is a function of the voltage on the belt. This voltage is typically formed with a charging corona that charges the photoconductive belt to a precalibrated level. However, as the characteristics of the belt, environmental factors, etc. change, the toner density also changes.
Previous solutions to the problem of varying toner density have primarily been directed toward measuring the toner density of a test patch and then comparing it to a predetermined value. Different voltages can be imparted to the photoconductive belt to vary the toner density of the patch, and then the voltage associated with the patch that most closely matches the desired toner density chosen as the operating voltage. This is stored in the control mechanism for the print engine. Subsequent copies made by the print engine will then utilize this voltage. Periodically, the test patch is again run and the voltage either changed or left alone.
One type of conventional toner density sensor is that utilizing infrared (IR) diodes and sensors that are operable to transmit infrared radiation onto a surface at an angle thereto, and then sense the reflected light energy. One type of sensor is disclosed in U.S. Pat. No. 4,652,115, issued to Palm, et al. on Mar. 24, 1987, and assigned to the present assignee. One problem that exists with use of this type of sensor is the signal-to-noise ratio that degrades significantly when trying to determine the density of a patch of black toner that is deposited directly on the surface of the transfer belt. The transfer belt is typically a dark color and, even though the radiation is at infrared wavelengths, a significant portion of this is absorbed by the underlying belt transfer, such that sufficient energy is returned to the sensor to provide reliable measurements. When measuring toner densities utilized in color reproduction, this does not present a problem. It is only with respect to the black toner that the measurement of toner density suffers from signal-to-noise problems.
In view of the above disadvantages, there exists a need for an improved method for monitoring the toner density for black toner, especially in a multi-color print engine utilizing a black toner as one of its primary colors.